Glaucoma is a disease characterized by elevated intraocular pressure which, if not checked, may lead to nerve damage and visual loss. Pressures in the range of from about 15.+-.3 mm. Hg. up to about 21 mm. Hg. may be considered to be in the normal range for human beings, whereas pressures substantially above that range are considered abnormally high. If pressures in the higher range are maintained for substantial periods of time, damage to the optic nerve of the eye may occur, leading to a narrowing of the field of vision and eventually to blindness if not appropriately treated. Although in certain cases glaucoma can be treated through the administration of certain medicines such as pilocarpine, epinephrine and timololmaleate, it is often necessary to surgically provide for the release of intraocular pressure for those patients who do not respond to drug therapy or who continue to lose vision under therapy.
Medical researchers have investigated a number of methods for the surgical release of intraocular pressure. Such surgery, in its simplest form, has involved making a small, surgical incision into the anterior chamber at or near the limbus so as to provide means for releasing an overabundance of aqueous humor from the eye into an adjacent subconjunctival space and thus to lower the intraocular pressure. In a modification of this procedure, a hair or other wicking material is reported to have been placed in the incision to provide a continuous passageway for excess fluid to be discharged from the eye. Other researchers have implanted small tubes that extend through the eye wall at the limbus or scleral-corneal junction for the purpose of providing a channel through which aqueous humor can escape. Such surgical procedures, although still used to some extent, are far from adequate. Healing of the subconjunctival drainage space frequently results in scarring, rendering the space non-absorbent of aqueous humor. When this occurs, no liquid flow through the eye wall occurs, and the intraocular pressure may hence rise to dangerous levels.
An excellent account of the history of glaucoma surgery is found in Bick, Use of Tantalum for Ocular Drainage, Archives of Ophthalmology 42:373-388 (1949).
In a recent embodiment, the exterior end of a tube extending through the wall of the eye is provided with a pressure relief valve in the form of small slits made through the wall of the tube at its end. Reference is made to Krupin, T., et al, Valve Implants in Filtering Surgery, Am. J. Ophthmol. 81:232-235, 1976. It is reported that fairly close control over the pressure needed to open the valve may be obtained. If the exterior or distal end of the tube is inserted beneath a flap of conjunctiva or the like, of course, the valved tube is subject to the same drawbacks as the other tubes described above.
Glaucoma surgeons have discovered that when surgery fails it is usually because the "bleb", the subconjunctival drainage space created by the surgeon, has become fibrosed, causing it to shrink and become non-absorbing.
One device that has been somewhat successful in maintaining the fluid absorbancy of the bleb during the healing process was described by Molteno in 1969 Molteno, "New Implant for Drainage in Glaucoma," British Journal of Opthalmology, Vol. 53, p. 161 (1969) described a device made from a "stellon" brand acrylic monomer. The device consisted of two parts--a flat plate fashioned to conform to the sclera and a gutter incorporated at the point where a drainage tube met the plate to assure an even spread of drainage into the bleb. In 1979, Molteno disclosed a new device that had a biconcave base plate and a long silicone tube, which served the same function as the first device. Reference is made to Chapter 11 of Glaucoma Surgery by Luntz, M. H., Harrison, R. and Schenker, H. I. (1984) for a description of this device.
There is a need in the medical field for a tissue-implantable device which would operate substantially on a continuous basis to permit excess aqueous humor to drain from the eye but would not be subject to the drawbacks associated with healing and scarring of tissue.